One technique commonly used in optical and other communication systems to maximize the amount of data that can be transmitted and/or received per unit time (i.e., bandwidth) is frequency division multiplexing. In frequency division multiplexing, separate data signals are modulated onto different wavelengths and transmitted simultaneously over the same fiber. A plurality of data signals carried on different light wavelengths (hereinafter sometimes referred to as sub-carriers or sub-carrier channels) can be simultaneously transmitted on the same fiber without the signals on the different wavelengths interfering with each other and can be separated from each other at a receiving end using filters and/or other techniques. One of the advantages of fiber optic communication systems is that the lasers used therein to transmit the data generate light in extremely well-contained and narrow wavelength bands, thus facilitating the use of frequency division multiplexing. It often is desirable for the wavelength channels to be as close to each other as possible without interfering with each other in order to maximize the number of different wavelength channels in a given communication network. On the other hand, there is some wavelength diversity or spread in any designated wavelength carrier. The spread can be due to many, factors, but is primarily due to the modulated information signal. Accordingly, in optical frequency division multiplexing, the spacing between adjacent carrier wavelengths should be selected in order to avoid or minimize cross-talk between the signals in two different wavelength channels.
In any event, most, if not all, fiber optic communication systems employ optical band pass filters to block noise outside of the wavelength range that is used for communications in the system. Typically, the band defined by such optical band pass filters is relatively tight around the wavelength range supported by the system. Thus, in a system using frequency division multiplexing, the cutoff frequency at the low end of the optical band pass filter is relatively close to the lowest frequency sub-carrier and the upper cutoff of the optical band pass filter is relatively close to the highest frequency sub-carrier supported by the system.
For instance, FIG. 1A is a graph of optical power as a function of optical frequency (1/wavelength) for an exemplary communication system using two sub-carrier wavelengths. The spectrum of the first, lower sub-carrier frequency is represented by trace 101 and the spectrum of the second, higher frequency sub-carrier is represented by trace 103. The frequency response of the optical band pass filter is represented by trace 105. FIG. 1A shows a relatively ideal situation in which the frequency envelope of the first and second carrier frequencies 101, 103 are spaced far enough from each other that there is very little frequency overlap between the two channels (as seen within area 107 of the graph) and the full spectrums of the two channels are virtually fully contained within the frequency envelope 105 of the optical band pass filter. In this example, there will be virtually no cross-talk between the two sub-carriers and virtually all of the power of the sub-carriers 101, 103 will pass through the optical filter.
Turning now FIG. 1B, this figure illustrates a situation in which the frequencies of the two sub-carrier channels 101, 103 are too closely spaced to each other such that there is significant overlap of the frequency spectrums of the two channels (as seen within area 108). This will cause significant cross-talk between the two sub-carriers, resulting in poor communication quality (i.e., a high bit error rate (BER)) due to interference between the two channels.
FIG. 1C illustrates a contrasting problem. Particularly, FIG. 1C illustrates a situation in which the sub-carrier channels 101, 103 are spaced too far apart. As can be seen in FIG. 1C, there is no cross-talk, but a substantial amount of the spectrum of each of the sub-carriers is outside of the band 105 of the band pass filter (as seen within areas 109 and 110), thus resulting in the loss of a substantial amount of data (i.e., a high bit error rate (BER)).